1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display device, and particularly to a method of manufacturing a liquid crystal display device using a normal-pressure dispensation alignment method.
2. Description of the Prior Art
As a display device of an audio-visual machine and an office automation machine, a liquid crystal display device (LCD) has been widely used because of its merits including thin thickness, a light weight, a low power consumption and the like. With this liquid crystal display device (hereinafter abbreviated to “LCD”), liquid crystal is filled into a gap between a pair of substrates which face each other. An electric field made of electrodes provided onto each of the substrates controls the alignment direction of liquid crystal molecules, and thereby modulating beams of light which are irradiated onto the LCD. Thus, the LCD displays an image.
As methods for filling liquid crystal into the LCD, the following methods have been generally known.
According to a first conventional method of filling liquid crystal, first of all, substrates facing each other are aligned with sealing material except for an area which are assigned as a port through which liquid crystal is filled (hereinafter, the port will be referred to as a “filling port”). Subsequently, the aligned substrates are placed into a vacuum container so that the aligned substrates are put under a low pressure condition. Thereafter, the pressure in the vacuum container is returned to an atmospheric pressure while the filling port remains to be soaked in liquid crystal. This is what is called a method employing difference in pressure.
According to a second conventional method of filling liquid crystal, substrates facing each other are aligned with sealing material except for areas which are assigned as a filling port and a port through which air is exhausted (hereinafter, a latter port will be referred to as an “exhausting port”). Thereafter, the filling port is soaked into liquid crystal, and air is exhausted through the exhausting port, and thereby liquid crystal is sucked up thereinto.
According to a third conventional method of filling liquid crystal, in an atmosphere whose pressure has been reduced by a vacuum container or the like, sealing material is coated, and liquid crystal is dispensed, on one of the two substrates, and thereafter the other of the two substrates is aligned thereon. Subsequently, the pressure under which the substrates are being placed is returned to the atmospheric pressure so that the sealing material is cured (hereinafter, this method will be referred to as a “vacuum dispensation alignment method.” See Japanese Patent Laid-open Official Gazette No. 2002-318378, for example).
According to a fourth conventional method of filling liquid crystal, in the atmospheric pressure, sealing material is coated, and liquid crystal is dispensed, on one of the two substrates, and thereafter the other of the two substrates is aligned thereon. Subsequently, the sealing material is cured while the pressure is being increased (hereinafter, this method will be referred to as a “normal-pressure dispensation alignment method”).
The methods for filling liquid crystal by use of difference in pressure or exhaustion require a filling port to be sealed with resin or the like after liquid crystal is filled. Accordingly, this brings about problems that bubbles are mixed into the liquid crystal through the filling port, and that liquid crystal is contaminated with the sealing material.
The vacuum dispensation alignment method can reduce time needed for filling liquid crystal to a large extent, and can reduce an amount of valuable liquid crystal needed for manufacturing in comparison with the methods for filling liquid crystal. However, larger facilities are required for performing a process of dispensing liquid crystal, and a process of aligning substrates, under a low pressure condition. In addition, the vacuum dispensation alignment method brings about a problem of deterioration in reliability of an LCD for the following reason and the like. Dispensed liquid crystal spreads so rapidly that the liquid crystal comes into contact with the sealing material before the sealing material has dried up. Accordingly, ingredient materials of the sealing material are dissolved into the liquid crystal. By contrast, the normal-pressure dispensation alignment does not require a pair of substrates to be maintained under a low pressure condition, thereby enabling the manufacturing facilities to be simplified. In addition, the spread of the liquid crystal can be controlled easily, and can be checked visually. Accordingly, the normal-pressure dispensation method has a feature of better workability.
Here, descriptions will be provided for the conventional normal-pressure dispensation alignment method with reference to FIGS. 1, and 2A to 2G.
FIG. 1 is a flowchart showing parts of a method of manufacturing an LCD using the conventional normal-pressure dispensation alignment method. FIGS. 2A to 2G illustrate a cross-sectional process flow schematically showing a structure of the substrate.
First of all, a TFT substrate 2 and a counter substrate 3 are prepared. On the TFT substrate 2, switching elements such as Thin Film Transistors (TFTs) have been formed. On the counter substrate 3, color filters, black matrixes and the like have been formed. Before alignment layers 4 are formed respectively on the TFT substrate and the counter substrate, wet cleaning by use of detergent, an organic solvent, pure water and the like as well as dry cleaning by use of ultraviolet radiation (101a and 101b) are performed on both the TFT substrate and the counter substrate. After these cleanings, the substrates are dried up by use of a hot plate, an IR heating machine and the like (102a and 102b).
Subsequently, the surfaces of the respective substrates are coated with a polyimide solution which is material for alignment layers 4 by use of a printing machine (103a and 103b). Thereafter, the substrates are temporarily baked at a temperature of around 80° C. by use of a hot plate, an IR heating machine or the like. Then, complete baking for shrinking the layers coated with a polyimide solution by heating to be dehydrated is performed (104a and 104b) so that the alignment layers 4 are formed with an even thickness (as shown in FIG. 2A).
Subsequently, for the purpose of controlling an alignment direction of the liquid crystal, rubbing processes are performed respectively on the surfaces of the respective post-baked alignment layers 4 in a way that the surfaces of the respective post-baked alignment layers 4 are rubbed in one direction only by use of buff cloth which has been rolled up around a rotating metallic rollers (105a and 105b). In this occasion, as shown in FIG. 2B, residues 5 such as waste pieces of thread of the buff cloth and shavings of the alignment layers 4 stick to the surfaces of the respective substrates.
Then, in order to remove the residues 5 from the surfaces of the respective substrates, the substrates are cleaned by an ultrasonic cleaning method, a jet spray method or the like, those of which use hot ultra pure water and alcohol and the like (106a and 106b). With regard to these cleaning steps, restrictions are imposed on the cleaning methods in order to avoid damaging the alignment direction of the alignment layers. For this reason, residues 5 which have been unable to be removed may remain on the surfaces of the respective substrates in some cases (as shown in FIG. 2C). Subsequently, the substrates are put into a drying furnace so that the substrates are dried up by hot air circulation, an IR heating machine or the like (107a and 107b).
Thereafter, one of the two facing substrates (the TFT substrate 2 in this case) is coated with sealing material 6 made of ultraviolet curing resin, thermal curing resin and the like by use of a screen printing method, a dispenser writing method and the like. In addition, the substrate is coated with Ag (109). By use of a wet spraying method or a dry spraying method, spacers 7 such as polymer beads and silica beads are sprayed onto the other substrate (the counter substrate 3 on this case), and the spacers 7 are caused to adhere onto the substrate (110) (as shown in FIG. 2D). In this occasion, depending on a condition in which the spacers 7 are sprayed, the spacers 7 are not distributed adequately, and coagulate so that coagulations of spacers 7a may remain on the substrate in some cases.
Subsequently, by use of a dispenser for dispensing liquid crystal or the like, liquid crystal 8 is dispensed in an adequate amount in a display area surrounded by the sealing material 6 on one of the two substrate (the TFT substrate in this case) in a normal pressure (as shown in FIG. 2E). Thereafter, the other substrate (the counter substrate in this case) is positioned and aligned thereto (as shown in FIG. 2F). Then, the pair of substrates are pressed against each other from the outsides of the respective substrates so that the sealing material 6 is squashed, thereby forming a desired gap between the two substrates. Thence, the sealing material 6 is temporarily cured by irradiating beams of ultraviolet light or the like onto the sealing material 6 from the back of the substrate (the TFT substrate in this case). Thereafter, the sealing material 6 is heated at a temperature of around 120° C., and thereby is completely cured (114) (as shown in FIG. 2G). Subsequently, the pair of substrates are cut along a predetermined portion outside the sealing material 6 (115). The LCD 1 is formed through the aforementioned processes.
However, the aforementioned method of manufacturing an LCD using the normal-pressure dispensation alignment method brings about the below-described problems.
A first problem is deterioration in reliability of a liquid crystal panel, which is caused by the following mechanism. According to the normal-pressure dispensation alignment method, each process is performed in the atmosphere. For this reason, residues, for example, inorganic ions such as Na+, K+ and Cl− as well as organic matters such as phthalate esters and cyclic siloxane, stick onto the surfaces of the respective substrates. For example, if inorganic ions would be mixed into the liquid crystal 8, leak and the like would be caused. This would bring about visible defects, such as reduction in voltages applied to the respective common electrodes and the like as well as stain to be visualized in a periphery of the LCD.
A second problem is reduction of yields, which is caused by the following mechanism. The conventional normal-pressure dispensation alignment method does not have a step of cleaning the surfaces of the respective substrates immediately before liquid crystal is dispensed. Accordingly, residues 5, such as waste pieces of thread of the buff cloth and shavings of the alignment layers, which have been unable to be removed, and foreign object, such as coagulations of spacers 7a, which have stuck to the surfaces of the respective substrates in the step of spraying spacers, may remain in some cases. In each of portions into which the residues 5 and the coagulations of spacers 7a have been mixed, the thickness of liquid crystal 8 is reduced, thereby causing visible defects such as defects of visualized bright dots (what is called bright defects).
A third problem is another reduction in yields, which is caused by the following mechanism. In a heating step of completely curing the sealing material 6, organic matters which have been formed on the surfaces of the respective substrates (for example, alignments layers 4) may be activated, thereby causing migration. Accordingly, for example, the alignment direction of the alignment layers 4 is put out of order, thus causing visible defects such as visualized bright defects (which is called alignment brightness).
The present invention has been made with the aforementioned problems taken into consideration. A first object of the present invention is to provide a method of manufacturing an LCD whose reliability and yields can be improved by securely removing inorganic ions, dust including organic matters, residues and contaminant particles such as coagulations of spacers which have stuck to the substrates facing each other so as to reduce visible defects.
In addition, a second object of the present invention is to provide a method of manufacturing an LCD whose yields can be improved by inhibiting organic matters, which have been formed on the substrates, from activating so as to reduce visible defects.